Calibration fault detector and automatic calibrator

ABSTRACT

An automatic calibrator is provided in combination with a wheel aligner used in determining the toe angle and camber angle of a wheel. A target is connected to the wheel aligner to move therewith. When the wheel aligner is at a predetermined angle, the target is used to provide a signal indicating that the wheel aligner is at the predetermined angle. A transducer output signal is then checked to determine if the transducer output signal accurately corresponds to the predetermined angle.

This is a division of application Ser. No. 06/129,269, filed Mar. 10,1980 now abandoned.

TECHNICAL FIELD

The present invention relates to fault detecting and self calibratingdevices and, in particular, to an apparatus and a method for detectingwhen a wheel angle measuring apparatus is not properly calibrated andfor correcting the calibration error.

BACKGROUND ART

There are a variety of instruments used to measure predefinedparameters. Many of these instruments are calibrated after assembly andtested for proper calibration by using a test sample which has knownparameters. The parameters of the test sample are measured by theinstrument. If properly calibrated, the instrument provides anindication of the known parameters. If not properly calibrated, theinstrument outputs data different than the known parameters. Themeasuring instrument can then be manually calibrated to correctlyreflect the known parameters.

A testing apparatus which includes a measuring system is disclosed inU.S. Pat. No. 3,187,440 to Merrill et al. entitled "Dynamic WheelAlignment Testing Apparatus." The wheel aligner described thereinautomatically checks and measures specified interrelated angles whichare pertinent to aligning the front wheel suspension system of avehicle. Among such angles, toe in is defined as the inward slanting ofthe wheels toward the front while camber is defined as the inwardsloping of the wheels toward the bottom. The caster and kingpininclinations are mathematically related to the camber angle of thewheel. The measuring system provides a visual indication of each of therelevant inclinations; however, it is assumed that the measuring systemis properly calibrated prior to determining the various angles. That isto say, it is expected, if the toe display meter indicates an angle ofzero degrees, that there is no inward slanting of the wheels toward thefront. However, if signals from the electronics circuitry which controlmeter movement are altered or have drifted from initial referencelevels, the toe display meter may register a value other than zerodegrees even though the angle should provide a reading of zero degrees.Such discrepancies between the visual meter indication and the actualangles pertinent to the front wheel suspension system result in thedetermination of inaccurate wheel alignment data and the making ofincorrect wheel alignments.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus for minimizinginaccurate measurements and, more specifically, to minimizingcalibration errors in a calibration fault detecting and automaticcalibrating apparatus which is responsive to a monitored apparatus. Fromthe following discussion, it will be readily appreciated that thepresent invention can be used in combination with a number of monitoredapparatus. For example, the automatic calibrating apparatus describedherein could be used to minimize inaccurate lathe machine positions sothat the machine properly works material in a predefined manner. In theembodiment described herein, the present invention is used incombination with a wheel aligning apparatus for minimizing theoccurrence of inaccuracies when measurements are made of the variousinterrelated angles which are significant in a front wheel suspensionsystem. If an angle measurement is provided which is incorrect in viewof the sensing of a preestablished reference angle or position, theapparatus of this invention provides a visual indication that thismeasurement is inaccurate. Based on this indication, an operator canrecalibrate the apparatus so that the angle provided and displayed on ameter corresponds to the actual angle of the wheel being measured. Withregard to measurement errors in which a measurement other than zerodegrees is given when the toe angle or camber angle of a wheel isactually at zero degrees, the apparatus of this invention automaticallyrecalibrates or corrects itself provided that the error or discrepancyis within predetermined limits.

The apparatus of the present invention includes transducers whichsimultaneously provide angle signals representative of the toe angle andcamber angle of the vehicle wheel. Each of the angle signals isoutputted to a checking circuit and subsequently gated to a meterdisplay which provides a visual indication of the toe and camber anglesof a wheel as it is dynamically tested. When the wheel alignment testingapparatus passes through points indicative of zero degrees toe orcamber, a control circuit sends a zero target pulse to a checkingcircuit. The checking circuit makes a determination as to whether theangle signal matches a predetermined output indicative of the zerodegree toe or degree camber. If there is not a substantial match, thechecking circuit outputs a signal for controlling an adjusting circuit.The adjusting circuit outputs a correcting signal for adjusting theangle signal so that the angle signal corresponds to the actual zerodegree value of the toe or camber angle. As a consequence, each meterdisplay for the toe and camber angles is corrected so that itcorresponds to the actual angle and the actual position of the wheel asdetermined by a reference circuit which monitors the position of thewheel aligning apparatus and the wheel being tested. The control circuitalso sends a target pulse to an enabling circuit which provides a visualindication as to whether the transducer outputs and meter displaycorrespond to the actual zero degrees value of the toe angle or camberangle.

The apparatus of the present invention also checks for propercalibration at a toe angle and a camber angle other than zero degreesand which is defined as the range point. Conveniently, an angle of apositive one degree is chosen for the range point. The control circuitprovides a positive one target pulse each time either the toe angle orcamber angle has a value of a positive one degree as determined by thereference circuit which monitors the position of the wheel aligningapparatus. The positive one target pulse is gated to an enabling circuitfor providing a visual indication as to whether or not the anglesignals, corresponding to the sensed toe or camber angle, match theactual one degree angle for the toe or camber of the wheel as providedby the reference circuit.

More particularly, an automatic calibrating apparatus is provided foruse in combination with an apparatus which is being monitored thereby,such as a wheel alignment testing apparatus. The automatic calibratingapparatus of this invention includes a reference circuit having a targetwhich is moveably positioned such that, whenever the wheel alignerpasses through points corresponding to an angle of zero degrees or onedegree for toe or camber, a change in a target signal outputted from thereference circuit occurs. Provided that an angle signal is withinpredetermined limits, a control circuit then produces a target pulsefrom a flip-flop arrangement. The angle signal corresponds to themeasured toe angle or camber angle of the wheel being tested. The anglesignals outputted by transducers which monitor the position of the wheelaligning apparatus are gated to a checking circuit. When the change inthe target signal from the reference circuit is indicative of a zero orpositive one toe angle or a zero or positive one camber angle, thesample and hold circuit latches the angle signal for a predeterminedtime period. When the change in the target signal is indicative of azero degree toe or camber and if the angle signal does not correspond toa zero degree value, a correcting signal is generated by an adjustingcircuit. The adjusting circuit includes an up/down digital counterenabled by a control circuit target pulse. The digital counter outputs adigital signal indicative of the magnitude of the error in the anglesignal. This digital signal is inputted to a digital-to-analog converterwhich outputs a correcting signal. The correcting signal is applied to asumming amplifier in the checking circuit, together with the anglesignal, so that the target of the summing amplifier is a corrected anglesignal corresponding to the actual zero degrees for angle or camberangle.

If the target pulse from the control circuit is generated because thewheel aligning apparatus and the wheel being tested are passing througha range point of one degree toe or one degree camber, the digitalcounter is not enabled. But this positive one degree target pulse isgated to an enabling circuit and indicators, such as light emittingdiodes (LEDs), which output an indication of whether the actual rangepoint of one degree as provided by the reference circuit equals thevalue of the corrected angle signal. Similarly, after the correctingsignal is generated by the adjusting circuit and summed with the anglesignal, the zero degree target pulse permits the enabling circuit toprovide an indication of whether the corrected angle signal correspondsto zero degrees toe or zero degrees camber as indicated by the referencecircuit.

From the foregoing description, a number of worthwhile advantages of thepresent invention are readily discerned. A fault detecting and automaticcalibrating apparatus is provided for use in combination with amonitored apparatus. The calibrating apparatus checks and measures theposition of the monitored apparatus and determines whether the measuredposition corresponds to a preestablished reference position of themonitored apparatus. If there is a lack of correspondence, thecalibrating apparatus adjusts itself so that the measured position doescorrespond. Furthermore, the calibrating apparatus provides a visualindication as to whether the calibrating apparatus is properlycalibrated at two separate points corresponding to two differentpositions of the monitored apparatus. As a consequence, if acorrespondence results at the first position of the monitored apparatusbut is not present at the second position, an indication thereof isprovided so that the calibrating apparatus can be corrected. As a resultof these features, correct measurements of the position of the monitoredapparatus are provided.

In the embodiment described herein, the fault detecting and automaticcalibrating apparatus responds to a wheel aligning testing apparatus. Ifthe calibrating apparatus determines that an angle signal representativeof the toe or camber angle of a wheel being tested is different than apredetermined output when the alignment tester passes throughpreestablished reference positions indicative of zero degrees toe orzero degrees camber, the apparatus corrects the angle signal so that aproper indication of zero degrees is given on a meter display. If theangle signal is not properly corrected when the alignment tester is atzero degrees toe or camber, an indication is provided that the apparatusis not in proper calibration and should be manually calibrated beforecontinuing with the alignment procedure. Similarly, if the calibratingapparatus determines that the toe angle or camber angle as representedby the angle signals does not correspond to a positive one degree, anindication thereof is provided so that an operator can properly servicethe calibrating apparatus. These features greatly minimize thepossibility of inaccurate wheel alignment data being taken. Hence,unwarranted alignment changes are not made. Furthermore, when thecalibrating apparatus is used in facilities which record the variousinterrelated wheel angles, the recordings satisfactorily reflect theactual toe and camber angles and do not include errors resulting fromimproper apparatus calibration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the present invention connected to amonitored apparatus;

FIG. 2A is a schematic of the reference circuit including the target forconnection to the monitored apparatus;

FIG. 2B is a perspective view of the target and photosensors showing thehigh light reflecting surfaces and the low light reflecting surfaces ofthe target;

FIG. 3 is a schematic showing the integrating circuit and checkingcircuit;

FIG. 4 is a schematic showing the control circuit;

FIG. 5 is a schematic illustration of the adjusting circuit;

FIG. 6 is a schematic showing the enabling circuit and LED indicators;

FIGS. 7A-7H illustrate a signal waveform timing diagram showing thegeneration of the target signal and target pulses; and

FIGS. 8A-8H illustrate a signal waveform timing diagram associated withthe control circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to FIG. 1, a block diagram of the present inventionis provided. A monitored apparatus 10 is monitored by the faultdetecting and automatic calibrating apparatus. The monitored apparatus10 can include a number of different apparatus. In the embodimentdescribed in detail herein, the monitored apparatus is a wheel aligningtesting apparatus. However, it is readily understood that the faultdetecting and automatic calibrating apparatus is not confined to usetherewith. The monitored apparatus can be a lathe-related machine inwhich the fault detecting and automatic calibrating apparatus is used tominimize inaccuracies in the working of materials by the machine. Themonitored apparatus could also be further defined as a light beam, suchas a laser beam, in which the position thereof at predetermined timeperiods is a salient feature for proper operation. Consequently, it canbe appreciated that, although the present invention will be described inrelation to a wheel alignment testing apparatus, its use is not limitedthereto.

The monitored wheel aligning apparatus 10 moves simultaneously aboutboth a horizontal axis and a vertical axis in response to forces appliedby a wheel being tested by the apparatus 10. The apparatus 10 is thealigning apparatus disclosed in U.S. Pat. No. 3,187,440 to Merrill etal. Transducers 12 monitor the movement of the wheel aligning apparatus10. A first transducer monitors the toe angle of the wheel and providesa signal indicative of the toe angle while a second transducer monitorsthe camber angle of the wheel and provides a signal indicative of thecamber angle. Toe-in is generally defined as the inward slanting of thevehicle wheel toward the front. Camber is generally defined as theinward sloping of the wheel toward the bottom. The transducers 12 arelinear variable differential transducers (LVDTs) which provide an ACsignal. The amplitude of the AC signal is proportional to thedisplacement of the core of the transducer 12 from a null or zero point.The greater the distance the wheel aligning apparatus 10 is from thezero toe angle or zero camber angle, the greater in absolute magnitudeis the AC signal. Each transducer is mechanically adjustable in order tomatch the null point of the transducer with the zero angle position ofthe wheel aligning apparatus 10. As a consequence, when the monitoringtransducers 12 output a signal corresponding to a zero degree toe orcamber, the monitored wheel aligning apparatus 10 is physicallypositioned at a zero degree toe or a zero degree camber. Each signalrepresenting the toe angle or camber angle of the wheel is amplified,rectified and integrated by an integrating circuit 14. Integratingcircuit 14 outputs an angle signal corresponding to the toe angle orcamber angle of the wheel. Further assurance that the toe or camberangle signals correspond to the actual position of the wheel aligningapparatus 10 is provided by the modifying circuit 16. When the monitoredwheel aligning apparatus 10 is located at zero degrees toe angle andzero degrees camber angle, the modifying circuit 16 adjusts eachtransducer signal so that an angle signal corresponding to zero degreestoe or zero degrees camber is outputted by the integrating circuit 14.In addition to modifying the transducer signal at zero degrees toe orcamber, the modifying circuit 16 also permits each transducer signal tobe adjusted so that it corresponds to a wheel aligning apparatusposition of a positive one degree toe or camber. This one degreeposition is defined as the range point.

A reference circuit 18 also separately monitors the position of thewheel aligning apparatus 10. A target signal is outputted by thereference circuit 18. Each time the wheel aligning apparatus 10 passesthrough a point corresponding to zero degrees toe angle or zero degreescamber angle, the magnitude of the target signal changes. The targetsignal is inputted to a control circuit 20. The angle signal fromintegrating circuit 14, representing the toe angle or camber angle, isgated to a checking circuit 22. The checking circuit 22 in conjunctionwith an adjusting circuit 24 corrects the angle signal so that itproperly reflects the position of the wheel aligning apparatus 10. Thechecking circuit 22 outputs a corrected angle signal corresponding tothe actual toe angle or camber angle of the wheel. This corrected anglesignal is applied to the control circuit 20. Each time the target signalchanges in magnitude when the wheel aligning apparatus 10 is at zerodegrees or a positive one degree and the magnitude of the correctedangle signal is within predetermined limits, a target pulse is generatedby the control circuit 20. A zero target pulse defines the presence ofthe wheel aligning apparatus at a reference position indicative of azero degree toe angle or zero degree camber angle. A range target pulsedefines the presence of the wheel aligning apparatus 10 at a referenceposition indicative of a positive one degree toe angle or a positive onedegree camber angle. Two different preestablished reference angles orpositions are checked to minimize inaccuracies which may occur when thetoe or camber angle signal does correspond to a first preestablishedreference position but does not correspond to a different preestablishedreference position.

Each target pulse is applied to the checking circuit 22. A sample andhold circuit in the checking circuit 22 responds to the target pulse sothat the angle signal has adequate time to be corrected by the adjustingcircuit 24. The adjusting circuit 24 is enabled by the zero targetpulse. When the zero target pulse is present and the angle signal outputof checking circuit 22 does not substantially correspond in magnitude toa predetermined output or reference voltage signal, the adjustingcircuit 24 generates a correcting signal to compensate for this lack ofcorrespondence. The correcting signal is fed back to the checkingcircuit 22 to correct the angle signal inputted thereto. During the timeperiod in which no zero target pulse is present, the adjusting circuit24 continuously inputs the previously generated correcting signal to thechecking circuit 22. Upon the occurrence of the next zero target pulse,the adjusting circuit 24 generates an updated correcting signal when themagnitude of the angle signal does not match the reference voltagesignal. The corrected angle signal is also applied to a meter display26. The meter display 26 provides a visual indication of the toe angleor camber angle of the wheel being tested.

Each target pulse and each of the toe and camber angle signals are alsoapplied to an enabling circuit 28. At the occurrence of each targetpulse and when the toe or camber angle signal corresponds to zerodegrees or a positive one degree, one of the light-emitting diodes(LEDs) 30 turns on or is lit for a short predetermined period of time.If the toe angle or camber angle signal does not correspond to zerodegrees or a positive one degree, the LED 30 remains continuously lit.As a consequence, an operator can readily determine whether the faultdetecting and automatic calibrating apparatus is properly calibrated sothat an accurate display of the toe angle or camber angle of a wheelbeing tested is provided.

The various circuits comprising this invention are illustrated in FIGS.2-6. The reference circuit 18 is depicted in FIGS. 2A and 2B. Thereference circuit 18 is provided to monitor the toe angle of the wheelbeing tested and another reference circuit 18 is provided to monitor thecamber angle of the wheel being tested. The reference circuit 18includes a target 32 which is connected to the monitored wheel aligningapparatus 10. The target 32 is movable together with the monitored wheelaligning apparatus 10 as it responds to wheel forces being appliedthereto. The target 32 includes a high light reflecting surface and asurface which has a low light reflecting capability. As can be seen inFIG. 2B in the preferred embodiment, the target 32 includes alternatingsurfaces of high light reflection 34 and low light reflection 36. A pairof photosensors 38, 40 are fixedly mounted adjacent the path of movementof the target 32, approximately 1/16th of an inch therefrom. Thetransition points at which the surface of the target 32 changes fromhigh light reflecting to low light reflecting are formed forcorresponding to preestablished reference positions of the monitoredwheel aligning apparatus 10. The transition points are formed forcorrespondence with the zero degrees, positive one-half degree, andpositive one degree toe or camber angle of the monitored wheel aligningapparatus 10.

As the target 32 moves from a negative camber or toe out angle toward apositive camber or toe in angle, phototransistor 42 of photosensor 38 isturned on by the light emitted from LED 44. The light from LED 44 isdirected against and reflected by surface 34 of target 32. The increasedcurrent through phototransistor 42 results in a voltage defined as alogic HIGH at the output of operational amplifier 46 since its voltageoutput is proportional to the current through phototransistor 42. Duringthe time phototransistor 42 is turned on, phototransistor 48 ofphotosensor 40 is essentially cut off since the light emitted by LED 50is not reflected by the low light reflecting surface 36. Hence, thevoltage output of operational amplifier 52 is at a voltage defined as ata logic LOW.

When the monitored wheel aligning apparatus 10 and the target 32 aremoved to the zero degree toe or camber position, the current throughphototransistor 42 diminishes since the target transition point isacross from the photosensors 38, 40 and the low light reflecting surface36 is now across from LED 44. Consequently, the voltage output ofoperational amplifier 46 becomes a logic LOW. Conversely,phototransistor 48 turns on so that the voltage output of operationalamplifier 52 increases. When the target 32 reaches the one-half degreeposition, the output voltages of the operational amplifiers 46, 52 againswitch states. The output of operational amplifier 46 becomes a logicHIGH while the output of operational amplifier 52 becomes a logic LOW.Similarly, the outputs of the operational amplifiers 46, 52 switchstates at a positive one degree toe angle or camber angle position. FIG.7B illustrates the signal levels of the operational amplifiers 46, 52during the movement of target 32 from a negative camber or toe in angleto a positive camber or toe out angle. It is apparent that if feed backresistors 54, 56 are identical in value, the output voltages of theoperational amplifiers 46, 52 will be equal at the transition points ofzero degrees, a positive one-half degree, and a positive one degree.

Two photosensors 38, 40 are provided to minimize any effects thatelectronic signal changes or signal drifts may produce. For example, theLEDs 44, 50 are connected in series so that any current change will beexperienced in both LEDs. As a consequence and assuming thatphototransistors 42, 48 have substantially the same characteristics, thecurrent through the phototransistors 42, 48 will change accordingly.Since the occurrence of each transition point is determined by thematching of voltage outputs of operational amplifiers 46, 52 and nottheir absolute voltage magnitudes, the change in current does not affectthe sensing of the transition points. In addition, the zero degree andpositive one degree transition points of the target 32 are formed suchthat their direction of change is the same. That is to say, the outputvoltage of operational amplifier 46 changes from a logic HIGH to a logicLOW at both the zero degree and positive one degree transition point, ascan be seen from FIG. 7B. Conversely, the output voltage of operationalamplifier 52 changes from a logic LOW to a logic HIGH at both of thesetransition points. The occurrence of the zero degree and positive onedegree target transitions are subsequently used in other circuits of thefault detecting and automatic calibrating apparatus. As a consequence,in order to further minimize the effects of electronic signal drift, theoutput voltages of operational amplifiers 46, 52 at these two transitionpoints will be substantially the same.

The output of operational amplifier 46 is applied to the non-invertingterminal of comparator 58. The output of operational amplifier 52 isapplied to the inverting terminal of comparator 58. Comparator 58outputs a target signal which changes in magnitude in response to thechanges in the voltage outputs of operational amplifiers 46, 52. Asillustrated in FIG. 7C, the target signal changes from a logic HIGH to alogic LOW at the zero degree transition point. Similarly, the targetsignal output of comparator 58 changes from a logic HIGH to a logic LOWat the positive one degree transition point. The target signal changesfrom a logic LOW to a logic HIGH at the positive one-half degreetransition point.

In addition to the reference circuit 18 monitoring the position of thewheel aligning apparatus 10, the transducers 12 generate a signalindicative of the toe angle or camber angle of the wheel. One transducer12 monitors the toe angle while a second transducer monitors the camberangle. The displacement of the transducer core is a function of theposition of the wheel aligning apparatus 10. As can be seen in FIG. 3,each transducer signal is inputted to individual amplifying networks 60,62. Both amplifying networks 60, 62 receive an AC signal whose amplitudeis proportional to the displacement of the transducer core. Theamplitude of the AC signal inputted to the amplifying network 60 and theamplitude of the AC signal inputted to the amplifying network 62 aresubstantially equal when the wheel aligning apparatus is at zero degreestoe angle or zero degrees camber angle. As the wheel aligning apparatus10 moves away from the zero degree positions, the amplitude of thesignal applied to the amplifying network 60 changes inversely withrespect to the amplitude of the signal applied to the amplifying network62. That is, when the wheel aligning apparatus 10 is moved towards apositive camber or toe in, the amplitude of the signal inputted toamplifying network 60 increases while the amplitude of the signalinputted to amplifying network 62 decreases. When the wheel aligningapparatus is moved towards a negative camber or toe out, the amplitudeof the signal inputted to amplifying network 60 decreases while theamplitude of the signal inputted to amplifying network 62 increases.

The outputs of amplifying networks 60, 62 are applied to rectifyingcircuits 64, 66, respectively. The output of rectifying circuit 64 is anegative signal while the output of rectifying circuit 66 is a positivesignal. The rectified signals are summed and this resulting signal isgated to an integrator network 68. Since the amplitudes of the rectifiedsignals from rectifying circuits 64, 66 are opposite in sense or sign,whenever the two rectified signals are equal the input to theintegrating network 68 is zero volts. When the two rectified signals arenot substantially equal indicating a wheel aligning apparatus 10position other than zero degrees, a signal equal to the differencebetween the two rectified signals is generated. The integrating network68 integrates the summed signal providing an output proportional to thetoe angle or camber angle of the wheel. This output is defined as theangle signal.

Modifying circuit 16 is connected to the integrating network 68 andincludes trimmer circuits 70, 72. The trimmer circuits are used toadjust the integrating input signal to zero volts when the wheelaligning apparatus 10 is at zero degrees toe angle or zero degreescamber angle. This adjustment is made after the transducers 12 aremechanically adjusted so that each transducer output indicates that thewheel aligning apparatus 10 is at the zero degree position. Trimmercircuit 72 is provided to null out any DC offset voltage that may bepresent in the integrating network 68 and to electrically adjust thetransducer output signals.

The angle signal outputted by the integrating network 68 is applied toan amplifying network 76 of checking circuit 22. There is a checkingcircuit 22 for detecting the toe angle and another checking circuit 22for detecting the camber angle. Amplifying network 76 communicates witha sample and hold circuit 78 which gates the amplified angle signal tovoltage follower 80. The angle signal is then summed with a correctingsignal by summing amplifier 82. The generation of the correcting signalwill be fully explained later. The output of the summing amplifier 82 isa corrected angle signal representing the toe angle or camber angle ofthe wheel in which the electronic signal has been adjusted to match theactual position of the monitored wheel aligning apparatus 10.

Each of the corrected toe angle and camber angle signals outputted bytheir respective summing amplifiers 82 is applied to the control circuit20. It is understood that a separate control circuit 20 is provided forthe toe angle signal and for the camber angle signal. Referring to FIG.4, control circuit 20 includes a full wave rectifying circuit 84 whichreceives the corrected angle signal from summing amplifier 82. The anglesignal is essentially a ramp signal proportional to the displacement ofthe wheel aligning apparatus 10, as illustrated in FIG. 8A. Although theangle signal is depicted as linear and symmetric about a vertical axis,the angle signal actually comprises a number of minute voltage steps asa result of the latching or holding of the integrated transducer signalby sample and hold circuit 78.

The rectifying circuit 84 rectifies the angle signal so that theresulting ramp signal is V-shaped. Amplifying circuit 86 reverses thepolarity of the V-shaped angle signal so that the angle signal ispositive throughout its duration. The angle signal representing the toeangle or camber angle of the wheel being measured is then applied toeach of three comparators 88, 90, 92. Both first comparator 88 andsecond comparator 90 are connected at their non-inverting terminals tothe angle signal. Third comparator 92 is connected at its invertingterminal to the angle signal. Reference voltages are applied to theinverting terminals of first comparator 88 and second comparator 90. Athird reference voltage is applied to the non-inverting terminal of thethird comparator 92. The reference voltages to each of the comparators88, 90, 92 are selected to generally coincide with the voltage signalscorresponding to zero degrees, one-half degree, and one degree toe angleor camber angle thereby providing windows at these preestablished anglepositions. The outputs of comparators 88, 90, 92 contribute to thecontrolling of the adjusting circuit 24 and the enabling circuit 28. Inaddition, the comparators 88, 90, 92 assure that the change in thetarget signal at the one-half degree position will be inhibited. Thesefeatures are subsequently discussed in detail.

The reference voltage to the inverting terminal of first comparator 88provides an input signal corresponding to 105% of the range pointvoltage. The range point voltage is defined as the voltage correspondingto a one degree toe or camber. The reference voltage to the invertingterminal of second comparator 90 provides an input signal correspondingto 75% of the range point voltage. The reference voltage to thenon-inverting terminal of third comparator 92 provides an input signalcorresponding to 25% of the range point voltage. When the angle signalis greater than 105% of the range point voltage, the output of firstcomparator 88 is a logic HIGH, the output of second comparator 90 is alogic HIGH, and the output of third comparator 92 is a logic LOW. Thelogic HIGH of first comparator 88 is applied to the divider network 94.The divider network 94 communicates with the inverting terminal ofcomparator 96. The logic HIGH of second comparator 90 and the logic LOWof third comparator 92 are joined at circuit point 98 which provides alogic OR output to the resistive reference network 100 which is tied tothe non-inverting terminal of comparator 96. Since the voltage inputresulting from the divider network 94 is greater in magnitude than thevoltage input resulting from the input applied to the non-invertingterminal of comparator 96 through the resistive reference network 100, alogic LOW is outputted from comparator 96. When the angle signal becomesjust less than 105% of the range point voltage, first comparator 88becomes a logic LOW, second comparator 90 remains a logic HIGH, andthird comparator 92 remains a logic LOW. The logic LOW of firstcomparator 88 is applied to the divider network 94. Since the voltagesignal now applied to the inverting terminal of comparator 96 is less inmagnitude than that applied to the non-inverting terminal of comparator96, a logic HIGH is outputted from comparator 96. When the angle signalbecomes just less than 75% of the range point voltage, first comparator88 remains a logic LOW while second comparator 90 becomes a logic LOW.The output of third comparator 92 remains a logic LOW. The logic LOWoutputs of second and third comparators 90, 92 provide a logic LOWoutput at circuit point 98. This logic LOW is applied to the resistivereference circuit 100. The output of the resistive reference circuit 100communicates with the non-inverting terminal of comparator 96. Sincethis output is now less in magnitude than the input to the invertingterminal of comparator 96 from divider network 94, comparator 96 againoutputs a logic LOW. When the angle signal becomes just less than 25% ofthe range point voltage which corresponds to a one degree camber angleor a one degree angle, the outputs of comparators 88, 90 remain a logicLOW while the output of comparator 92 becomes a logic HIGH. Circuitpoint 98 then becomes a logic HIGH also. This logic HIGH throughresistive reference network 100 provides a voltage signal to thenon-inverting terminal of comparator 96 which is greater in magnitudethan the voltage output of the divider network 94. As a consequence alogic HIGH is again outputted from comparator 96. The foregoingcomparator input and output signals are illustrated in FIGS. 8A-8G. Theoutput of third comparator 92 is also applied to NAND gate 102 and tothe adjusting circuit 24 as an activation signal for purposes to bediscussed later.

At the same time the angle signal is inputted to the control circuit 20,the target signal from reference circuit 18 is applied to a "D"flip-flop 104 of control circuit 20 through delay circuit 106 whichdelays the input to the flip-flop 104 approximately 1 millisecond. Aclock signal is applied to the clock input of the flip-flop 104. In thepreferred embodiment, the clock signal is a 5 KHZ signal, such asdepicted in FIG. 7E. Each time the clock signal rises the compliment ofthe target signal which is present on the data line of flip-flop 104 isoutputted from the compliment output (Q) of the flip-flop 104 to adifferentiating circuit 108.

As illustrated in FIGS. 7A, 7C, and 7D, the target signal changes inmagnitude at zero degrees, a positive one-half degree, and a positiveone degree. The clock pulse gates the target signal, including thesignal level change at these preestablished wheel angles, to theflip-flop 104 output. The output of the differentiating circuit 108 isan impulse signal each time the target signal changes state, asillustrated by FIGS. 7D-7G. The impulse signal from the differentiatingcircuit 108 is applied to NAND gate 110 after diode 112 removes thenegative going impulse which occurs at a positive one-half degree toeangle or camber angle, as seen in FIG. 7H. The occurrence of the zerodegree and positive one degree target impulse signals are used tocontrol the checking circuit 22 and enabling circuit 28. Although the Qoutput is gated to differentiating circuit 108, it is understood thatthe Q output could be used instead. In the embodiment in which the Qoutput is connected to the differentiating circuit 108, when themonitored wheel aligning apparatus 10 moves from a toe out or negativecamber angle position to a toe in or positive camber angle position, theimpulse signal is in a positive sense at zero degrees and a positive onedegree. As a consequence, these impulse signals are applied to NAND gate110. When the monitored wheel aligning apparatus 10 moves from a toe inor positive camber angle position to a toe out or negative camber angleposition, the impulse signals are then in a negative sense at zerodegrees and a positive one degree. As a consequence, these impulsesignals are now removed by diode 112. Although the impulse signalassociated with the positive one-half degree position is now a positivegoing impulse, it is not gated through NAND gate 110 because the outputof comparator 96 is a logic LOW at this time. The positive going impulsefor the zero degree and positive one degree positions then are generatedonly when the monitored wheel aligning apparatus 10 is moving in a firstdirection and not in a second or opposite direction.

Conversely, in the embodiment in which the Q output is connected to thedifferentiating circuit 108, when the monitored wheel aligning apparatus10 moves from a toe out or negative camber angle position to a toe in orpositive camber angle position, the impulse signal is in a negativesense at zero degrees and a positive one degree. On the other hand, whenthe monitored wheel aligning apparatus 10 moves from a toe in orpositive camber angle position to a toe out or negative camber angleposition, the impulse signal is in a positive sense at zero degrees anda positive one degree. In this embodiment, therefore, the positive goingimpulse signals for the zero degree and positive one degree positionsare generated only when the monitored wheel aligning apparatus 10 ismoving in the second direction.

NAND gate 110 is also responsive to the output of comparator 96.Normally, the output of NAND gate 110 is a logic HIGH since the impulsesignals indicating that the monitored wheel aligning apparatus 10 is atzero degrees or a positive one degree are not present. In addition, theoutput of comparator 96 is usually a logic LOW until the angle signals,representing the toe or camber of the wheel being measured, provide avoltage within predetermined limits of the voltage corresponding to zerodegrees or a one degree toe or camber, as seen in FIG. 8H. Each timethis occurs, a logic HIGH is applied to one of the inputs of NAND gate110. When the logic HIGH impulse signal is also inputted to the NANDgate 110, the NAND gate 110 changes state to a logic LOW. This logic LOWis applied to NAND gate 114 which outputs a logic HIGH. This logic HIGHchanges the output state of NAND gate 116 to a logic LOW. The logic HIGHoutput of NAND gate 114 is also applied to NAND gate 118. The output ofNAND gate 118 remains a logic HIGH since the zero hold and range holdinputs are normally a logic LOW. The zero and range hold inputs will bemore fully explained later. Since the voltage across capacitor 120cannot change instantaneously, when the output of NAND gate 116 becomesa logic LOW the logic HIGH output of NAND gate 118 charges capacitor120. When capacitor 120 reaches the voltage level of a logic HIGH, NANDgate 114 again changes state to a logic LOW since the target impulsesignal is no longer present at the input of NAND gate 110. As a resultof the foregoing logic circuits, NAND gate 114 outputs a positive goingtarget pulse of 25 millisecond duration whenever the target impulsesignals are present while the output of NAND gate 116 is a negativegoing target pulse of 25 millisecond duration.

The positive going target pulse from NAND gate 114 is applied to one ofthe inputs of NAND gate 122 of adjusting circuit 24, as seen in FIG. 5.The output of NAND gate 122 is normally a logic HIGH which inhibitsup/down digital counter 124. If the positive going target pulse fromNAND gate 114 is present and if the output of comparator 92 of controlcircuit 20 is a logic HIGH indicating that the angle signal is within25% of the range point voltage, NAND gate 122 switches states to a logicLOW. The fact that the output of comparator 92 is a logic HIGH indicatesthat a zero target pulse is present. This logic LOW signal enables theup/down digital counter 124.

At the same time the positive going target pulse is applied to NAND gate122, the negative going target pulse from NAND gate 116 of controlcircuit 20 is applied to NAND gate 126 of the checking circuit 22, asseen in FIG. 3. When the negative going target pulse is present, theoutput of NAND gate 126 is a logic HIGH. The NAND gate 126 outputcontrols the sample and hold circuit 78. The sample and hold circuit 78includes a switch 128 which opens when a logic HIGH is outputted fromNAND gate 126. During the period when no target pulse is inputted toNAND gate 126, clock 130, which outputs the clock signal illustrated inFIG. 7E, controls the opening and closing of the switch 128. The switch128 gates the angle signal from the integrating network 68 afteramplification by amplifying network 76, as previously discussed.

When a target pulse is present indicative of the monitored wheelaligning apparatus 10 positioned such that the toe angle or camber angleof the wheel is at zero degrees or a positive one degree, the switch 128is opened so that the angle signal then present from the transducer 12is held on capacitor 132. Since the target pulse is 25 milliseconds induration, the capacitor 132 holds the angle signal input to the voltagefollower 80 for 25 milliseconds. The output of voltage follower 80 isapplied to the summing amplifier 82. The angle signal is summed with acorrecting signal from the adjusting circuit 24 to provide a correctedangle signal which is sent to a meter display 26. The meter display 26provides a visual read-out of the toe angle or camber angle of the wheelbeing tested.

Referring again to the adjusting circuit 24 of FIG. 5 to discuss thegeneration of the correcting signal, it is seen that the toe angle orcamber angle signal is inputted to the non-inverting terminal ofcomparator 134. The output of comparator 134 is applied to the up/downdigital counter 124. As previously described, up/down digital counter124 is enabled whenever a logic LOW is applied thereto from NAND gate122. When up/down digital counter 124 is enabled, it begins counting ina positive direction if the output of comparator 134 is a logic HIGH. Ifthe output of comparator 134 is a logic LOW, up/down digital counter 124counts in a negative direction. Since the inverting terminal ofcomparator 134 is essentially at zero volts, digital counter 124 countswhenever the angle signal is different than zero volts. As aconsequence, the digital counter 124 outputs a digital countcorresponding to the difference between the reference voltage of zerovolts, which is applied to the inverting terminal of comparator 134, andthe actual magnitude of the angle signal, which is applied to thenon-inverting terminal of comparator 134 from the output of the summingamplifier 82.

The digital counter 124 output is sent to a digital-to-analog (D/A)converter 136 which outputs an analog correcting signal. The correctingsignal, as previously noted, is summed with the angle signal provided bythe transducers 12 so that the angle signal becomes zero volts. A zerovolt angle signal output corresponds to a zero degree toe or camber inthe preferred embodiment. It is readily understood that if an anglesignal of zero volts is not present when the zero target pulse occurs,the correcting signal adjusts the angle signal so that it does providean indication of zero degrees to the meter display 26. In addition, thecorrecting signal remains constant until the next occurrence of the zerotarget pulse. Hence, the same correcting signal continues to adjust theangle signal from the transducer 12 during the entire sweep of themonitored wheel aligning apparatus 10. It is also readily appreciatedthat the range target pulse, which occurs at a positive one degree toeor camber, does not change the magnitude of the correcting signal. Theactiviation signal to NAND gate 122 is a logic LOW when the range targetpulse is present so that the up/down digital counter 124 is not enabled.

Additional circuitry is also provided which enables an operator of theapparatus to hold the value of the toe angle or camber angle on themeter display 26 when the zero target pulse or range target pulse ispresent. Referring to the control circuit 20 of FIG. 4, a range holdsignal is inputted to NAND gate 118. Assuming that no target pulse isthen present, the output of NAND gate 118 remains a logic HIGH. When arange target pulse occurs, the output of NAND gate 110 becomes a logicLOW. The output of NAND gate 114 becomes a logic HIGH. The output ofNAND gate 116 becomes a logic LOW. The output of NAND gate 118 alsobecomes a logic LOW since both inputs thereto are a logic HIGH. Thelogic LOW output of NAND gate 118 holds the outputs of NAND gates 114,116 at a logic HIGH and a logic LOW, respectively. The logic LOW fromNAND gate 116 is gated to the NAND gate 126 of the checking circuit ofFIG. 3 so that the output thereof is held at a logic HIGH. This logicHIGH holds the switch 128 open so that the angle signal then present isheld on capacitor 132 and applied to summing amplifier 82. Consequently,meter display 26 continues to display the same angle signal.

It is readily seen in FIG. 4 that the range hold signal also acts toinhibit the generation of a zero target pulse. The range hold signal isapplied to the NAND gate 102. As a consequence, when the output ofcomparator 96 becomes a logic HIGH indicating that the angle signal iswithin 25% of the range point voltage or approaching the zero degreemark, a logic LOW is outputted from the NAND gate 102. This logic LOWinhibits the gating of the zero degree impulse through NAND gate 110.

Similarly, when it is desirable to hold the reading on the meter display26 which represents the toe angle or camber angle at the occurrence ofthe zero target pulse, the zero hold signal of the control circuit 20 isswitched from a logic LOW to a logic HIGH. When the zero target impulseis present at the input of NAND gate 110, the logic HIGH output fromNAND gate 114 and the logic HIGH output of the zero hold signal switchthe output of NAND gate 118 to a logic LOW so that the target pulseoutputs of NAND gates 114, 116 remain at a logic HIGH and a logic LOW,respectively.

In the preceding discussion, it was assumed that the target impulsesignals, representative of the fact that the monitored wheel aligningapparatus 10 was positioned at a zero degree or a positive one degreetoe or camber, occurred when the corrected angle signal corresponded tozero degrees or a positive one degree toe or camber. If the correctedangle signal does not correspond, an indication is provided by enablingcircuit 28. The enabling circuit 28 provides a visual indication ofwhether the corrected angle signal properly corresponds to the actualposition of the wheel aligning apparatus 10 at zero degrees and apositive one degree toe or camber. Referring to FIG. 6, the enablingcircuit includes full wave rectifying circuits 136, 138. Inputted toeach rectifying circuit 136, 138 is the corrected angle signal output ofsumming amplifier 82 of FIG. 3. The output of rectifying circuit 138 isan essentially V-shaped ramp signal corresponding to the displacement ofthe wheel aligning apparatus 10. The ramp signal has a negative polarityand is applied to the inverting terminal of comparator 140. A firstreference voltage V1 is applied to the non-inverting terminal ofcomparator 140. The first reference voltage V1 is selected to be anegative voltage just less than zero volts so that the comparator 140output is usually a logic HIGH. When the corrected angle signalcorresponds to zero degrees toe or camber, the output of comparator 140becomes a logic LOW. The output of rectifying circuit 136 is also sentto the non-inverting terminal of comparator 142. A second referencevoltage V2 is applied to the non-inverting terminal of comparator 142.V2 is a negative voltage greater than V1 in absolute magnitude. As aconsequence, the output of comparator 142 becomes a logic LOW prior tothe output of comparator 140 changing from a logic HIGH to a logic LOWwhen the corrected angle signal approaches a value corresponding to zerodegrees.

The output signal of comparator 140 communicates with the data input ofthe "D" flip-flop 144. The output of NOR gate 146 is applied to theclock input of the flip-flop 144. Each time the clock input rises, thedata line input is gated to the Q output of the flip-flop 144. The Qoutput controls the biasing of transistor 148 which in turn acts as aswitch in turning light emitting diode (LED) or indicator 150 on andoff. NOR gate 146 has two inputs. The first input is the output ofcomparator 142. The second input is provided by the negative goingtarget pulse from NAND gate 116 of control circuit 20. Whenever thetarget signal is a logic HIGH indicating that the wheel aligningapparatus 10 is not at zero degrees or a positive one degree, the secondinput to NOR gate 146 is a logic HIGH. The output of NOR gate 146 isthen a logic LOW. While this output is a logic LOW, the output thenpresent at Q of flip-flop 144 controls the state of LED 150. When thecorrected angle signal approaches a value corresponding to zero degrees,the output of comparator 142 becomes a logic LOW. When the zero targetpulse occurs, the NOR gate 146 changes state from a logic LOW to a logicHIGH. This logic HIGH charges capacitor 152. Upon charging to apredetermined voltage, transistor 148 is turned on. LED 150 is thenprovided with a current path. This change in state is also applied tothe clock input of flip-flop 144 which causes the data input to be gatedto the Q output of flip-flop 144. If the data input is a logic LOWindicating that the corrected angle signal is substantially equal to avalue corresponding to zero degrees, the Q output is a logic LOW. As aconsequence, after the zero target pulse is no longer present and theoutput of NOR gate 146 becomes a logic LOW, capacitor 152 dischargesthrough resistor 154. Upon sufficient discharge or approximately 50milliseconds later, transistor 148 is cut off and LED 150 is no longerlit. Therefore, a flashing LED 150 indicates proper calibration. If, onthe other hand, the data input to flip-flop 144 is a logic HIGH,indicating that the corrected angle signal is not equal to a valuecorresponding to zero degrees, the zero target pulse causes the logicHIGH to be gated to the Q output of flip-flop 144. The logic HIGH holdstransistor 148 on so that LED 150 stays lit. A continually lit LED 150indicates improper calibration. The operator is then on notice of afault being present that must be corrected.

Rectifying circuit 136 is provided to monitor the presence of the rangepoint of a positive one degree. In addition to receiving the correctedangle signal, the rectifying circuit 136 also responds to a referencevoltage input. The reference voltage is chosen such that the sum of thereference voltage and the corrected angle signal corresponding to onedegree substantially equals zero volts in the preferred embodiment. Thisvoltage sum is applied to the inverting terminal of comparator 156. Athird reference voltage V3 is applied to the non-inverting terminal ofcomparator 156. V3 is a negative voltage just less than zero volts.Consequently, the output of comparator 156 is a logic HIGH until thecorrected angle signal approaches the voltage corresponding to onedegree. The output of comparator 156 is sent to the data line of the "D"flip-flop 158. The clock input of flip-flop 158 responds to the outputof NOR gate 160. A first input to NOR gate 160 is the output ofcomparator 162. The output of comparator 162 responds to the output ofcomparator 142. If the output of comparator 142 is a logic HIGH, thefirst input to NOR gate 160 is a logic LOW. Hence, when a range targetpulse is applied to the second input of NOR gate 160, the output thereofbecomes a logic HIGH. However, when a zero target pulse is appliedthereto, the output of NOR gate 160 remains a logic LOW since the outputof comparator 160 is a logic HIGH.

If the range target pulse is present, the clock signal gates the datainput signal to the Q output of flip-flop 158. The output of NOR gate160 also turns on transistor 164 after sufficiently charging capacitor166. Upon turning on, transistor 164 provides a current path for LED168. If the data input signal is a logic LOW when the clock signalrises, this logic LOW turns off transistor 164 after capacitor 166 hassufficiently discharged. The flashing LED 168 indicates that the faultdetecting and calibrating apparatus is properly calibrated at the rangepoint. If the data input signal is a logic HIGH when the clock signalrises, the LED 168 remains lit. The continually lit LED 168 provides anindication that the fault detecting and calibrating apparatus is notproperly calibrated at the range point. It is also understood, if norange target pulse or zero target pulse is generated, that LEDs 150, 168will remain continuously turned off. The zero and range target pulsesmay not be generated because the angle signals inputted to comparators88, 90, 92 are not within the predetermined tolerances for providing thewindows of FIG. 8H when the impulse signals are present. As aconsequence, an operator is put on notice that the calibration errorsare even greater than when the LEDs 150, 168 are continuously lit.

The enabling circuit 28 further includes a delay circuit 170. The delaycircuit 170 responds to each target pulse which is inputted to theenabling circuit 28. The delay circuit 170 delays the target pulse apredetermined time period so that the angle signal is adjusted by thecorrecting signal before the clock signal gates the data input signal tothe Q output of flip-flops 144, 158. As a result of this time delay, theangle signal after correction, rather than before correction, is checkedto determine whether it corresponds to the position of the wheelaligning apparatus 10. Diode 172 acts as a shunt for the target signalwhen the target pulse is no longer present so that the change of stateof the target signal from a logic LOW to a logic HIGH is immediatelyapplied to NOR gates 146, 160.

Based on the foregoing description, a number of advantages of thepresent invention are readily discerned. A fault detecting and automaticcalibrating apparatus is provided which minimizes the effects of signaldrifts by providing a balancing photosensor circuit. The calibratingapparatus is used in combination with a monitored apparatus forminimizing inaccuracies in the display or recording of measured data.The calibrating apparatus checks itself at two different positions ofthe monitored apparatus. If there is a lack of correspondence at a firstpreestablished position, the calibrating apparatus adjusts itself toprovide the correct output. If there is a lack of correspondence at asecond preestablished position, a visual indication is given that thecalibrating apparatus is improperly calibrated.

In the embodiment for use with a wheel aligning apparatus, the faultdetecting and automatic calibrating apparatus responds to signalscorresponding to toe or camber angles of a wheel being tested. A targetis connected to the wheel aligning apparatus such that, when the wheelaligner is at a zero degree position or a positive one degree position,a change in a target signal is generated. At each of these positions, acheck is made as to whether the automatic calibrating apparatus outputcorresponds to the actual position of the wheel aligner. If a correctedangle signal from the calibrating apparatus does not correspond to thepositive one degree position, a visual indication is provided to informan operator that the calibrating apparatus is not properly calibrated.If there is a lack of correspondence between the angle signal output ofthe calibrating apparatus and a predetermined output corresponding tozero degrees, the angle signal is corrected so that it corresponds tothe actual position of the wheel aligner. If the corrected angle signaldoes not correspond to the zero degrees position, a visual indication isprovided to inform the operator that the calibrating apparatus is notproperly calibrated. Consequently, accuracy in aligning the front wheelsuspension system of a vehicle is maximized. Furthermore, uncertaintyand errors in the measuring and recording of the various pertinent wheelangles are minimized.

The invention has been described in detail with reference to aparticular embodiment thereof, but it is understood that variations andmodifications can be effected within the spirit and scope of thisinvention. It is also understood that the above described inventioncould be implemented, in total or in part, by means of appropriatemicroprocessor circuits and instructions.

We claim:
 1. A calibration fault detecting and automatic calibratingapparatus for use in combination with a wheel aligner for detectingwhether the apparatus is calibrated to correctly provide an outputcorresponding to the toe angle or camber angle of a wheel being tested,the calibration fault detecting and automatic calibrating apparatuscomprising:target means connected to the wheel aligner and moveabletherewith; reference means responsive to said target means for producinga target signal indicating whether the wheel aligner is at a firstpredetermined toe angle or camber angle; sensing means responsive to themovement of the wheel aligner for generating a first signalcorresponding to the toe angle or camber angle of the wheel beingtested; control means responsive to changes in the target signal forproducing a target pulse when the wheel aligner is at the firstpredetermined toe angle or camber angle; adjusting means responsive tothe target pulse for generating a correcting signal corresponding to thedifference between the first signal and a predetermined outputrepresentative of the first predetermined toe angle or camber angle; andchecking means responsive to the first signal and the correcting signalfor providing a corrected angle signal corresponding to the toe angle orcamber angle of the wheel being tested.
 2. The apparatus, as claimed inclaim 1, further including:means for indicating whether the automaticcalibrating apparatus is correctly calibrated.
 3. The apparatus, asclaimed in claim 2, wherein:said indicating means includes meansresponsive to the target pulse for enabling indicators which produce anoutput indicative of whether the automatic calibrating apparatus iscorrectly calibrated.
 4. The apparatus, as claimed in claim 2,wherein:said indicating means includes means for delaying the targetpulse so that the indication of whether the automatic calibratingapparatus is correctly calibrated is made after the corrected anglesignal is generated.
 5. The apparatus, as claimed in claim 1,wherein:said control means generates a target pulse when the wheelaligning apparatus is at a second predetermined toe angle or camberangle different than the first predetermined toe angle or camber angle.6. The apparatus, as claimed in claim 5, wherein:said adjusting means isenabled by the target pulse generated when the wheel aligner is at thefirst predetermined toe angle or camber angle and is not enabled whenthe wheel aligner is at the second predetermined toe angle or camberangle.
 7. The apparatus, as claimed in claim 1, wherein:the target pulseis generated at the first predetermined toe angle or camber angle whenthe wheel aligner is moving in a first direction while the target pulseis inhibited when the wheel aligner is moving in a second directionopposite the first direction.
 8. The apparatus, as claimed in claim 1,wherein:said target means includes a target having surfaces of highlight reflection and low light reflection.
 9. The apparatus, as claimedin claim 8, wherein:said reference means includes a sensor fixedlypositioned adjacent the path of movement of the wheel aligner and saidtarget such that, when said sensor is substantially across from atransition point between the high and low light reflecting surfaces, thewheel aligner is at the first predetermined toe angle or camber angle.10. The apparatus, as claimed in claim 8, wherein:said target includestwo layers of alternatingly positioned surfaces of high and low lightreflections and said reference means includes a pair of sensors fixedlypositioned such that, when said sensors are substantially across from atransition point between the high and low light reflecting surfaces, thewheel aligner is at the first predetermined toe angle or camber angleand the output signal of one of said sensors changes in a firstdirection while the output signal of the other of said sensor changes ina second direction, opposite the first direction.
 11. A calibrationfault detecting and automatic calibrating apparatus for use incombination with a wheel aligner for detecting whether the apparatus iscalibrated to correctly provide an output corresponding to either thetoe angle or camber angle of a wheel being tested, the calibration faultdetecting and automatic calibrating apparatus comprising:first means forproviding an indication whether the wheel aligner is at either a firstpredetermined toe angle or a first predetermined camber angle; secondmeans responsive to the movement of the wheel aligner for generating afirst signal corresponding to either the toe angle or camber angle ofthe wheel being tested; third means responsive to said first means forproducing a control signal when the wheel aligner is at either the firstpredetermined toe angle or the first predetermined camber angle; andfourth means responsive to the control signal for detecting whether thefirst signal corresponds to either the first predetermined toe angle orthe first predetermined camber angle when the control signal is presentto determine whether the automatic calibrating apparatus is correctlycalibrated.
 12. An apparatus, as claimed in claim 11, furtherincluding:adjusting means responsive to said fourth means for generatinga correcting signal corresponding to the difference between the firstsignal and a predetermined output representative of either the firstpredetermined toe angle or the first predetermined camber angle.
 13. Anapparatus, as claimed in claim 12, further including:checking meansresponsive to the first signal and the correcting signal for providing acorrected angle signal corresponding to either the toe angle or camberangle of the wheel being tested.